Method of forming metal alloys

ABSTRACT

A method of forming superplastic metal alloys by heating the alloy to a temperature at which it is superplastic and then forming the alloy, in which the formed alloy is heated to a temperature above the superplastic temperature range in order to render it resistant to deformation before it is removed from the forming tool.

United States Patent [72] Inventor Bernard Brian Hundy [50] Field of Search 72/364, Woodstock, England 344,342; 148/115 [2|] Appl. No. 808,229 22 Filed Mar. 18, 1969 1 References Cited f' J y 1971 I UNITED STATES PATENTS I 731 33: $13? 3,340,101 9/1967 Fields, Jr. et a1 148/115 3,420,717 1 1969 F 1d ,J. t l 148 11.5 32 Priority Mu.21,196a I s r e a I 3 Britain Primary ExaminerRichard J Herbst [31 13,678/68 Attorneys-Stowe & Stowell and Thomas J. Greer, Jr.

1 ABSTRACT: A method of forming superplastic metal alloys [54] METAL ALLOYS by heating the alloy to a temperature at which it is superplastic and then forming the alloy, in which the formed alloy is heated [52] US. Cl 72/364, to a temperature above the superplastic temperature range in 72/344, 148/] 1.5 order to render it resistant to deformation before it is removed [51 1 Int. Cl ..B2ld 26/00 from the forming tool.

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METHOD OF FORMING METAL ALLOYS This invention relates to a method of forming metal alloys that exhibit superplasticity.

An article entitled superplasticity in an Al-Zn Alloy by W. A. Backofen, l. R. Turner and D. A. Avery, which was published in the Transactions of the A.S.M., Volume 57, 1964 pages 980-990, discloses the concept of forming superplastic metals by fonning techniques borrowed from polymer and glass processing. Moreover, one such forming technique using fluid pressure is disclosed in US. Pat. No. 3,340,101 which describes a method of forming superplastic metal blanks against a die surface by heating the blank to a temperature at which it is superplastic and then pressure or vacuum forming the blank to the shape of the die.

Compared with vacuum or pressure forming of thermoplastic synthetic plastics material and compared with press forming of conventional metal blanks, forming of superplastie metal alloys normally tends to be a relatively slow operation principally because the critical strain rate of the material, above which it does not behave in a superplastic manner, should not be exceeded.

Moreover, there may be a considerable time delay before the formed superplastic metal can be stripped from the forming tool and handled. This time delay arises from the fact that whilst the formed metal is at a temperature at which it exhibits superplasticity it is exceptionally vulnerable to deformation, and it must therefore be allowed to cool before subsequent handling; because the forming tools act as a heat sink the cooling time is excessively long for commercially viable production.

According to the present invention after the superplastic metal has been formed, it is heated to a temperature above the range at which it exhibits superplasticity before it is stripped from the forming tool and/or subsequently handled.

The heating of the formed superplastic metal renders it much more resistant to permanent deformation than when it is at the forming temperature, and the formed superplastic metal can generally be heated to a temperature at which it is sufficiently resistant to permanent deformation more quickly than it would take to cool the metal to a temperature at which it had a similar resistance to permanent deformation. Moreover, in the normal case where the formed metal has a greater coefficient of thermal expansion than the forming tool, the subsequent heating expands the formed metal more than the tool and thereby tends to free the metal from the tool whereas cooling, in many cases, tends to shrink the formed metal onto the forming tool. Furthermore the subsequent heating has the additional advantage of maintaining the forming tool at a suitable forming temperature, whereas if the formed metal and hence the forming tool is cooled to render the formed metal sufficiently resistant to deformation, further time delays occur in production whilst the forming tool is being reheated to the forming temperature before a further blank or the like can be formed.

The invention will now be described solely by way of example with reference to the following example and to the accompanying drawings in which FIG. 1 shows a schematic cross section ofa vacuum forming apparatus, and

FIG. 2 shows a schematic cross section of a pressure forming apparatus.

EXAMPLE A specimen of eutectoid zinc-aluminum alloy in the form of a standard I-Iounsfield tensometer round tensile test piece, type No. 14, was subjected to flow stress testing at various temperatures in a Hounsfield tensometer at a crosshead speed ofO. l 25 inch per inch per minute.

The results of the tests are summarized in the table:

Temperature Initial Flow Stress 270 641 pounds per square inch 290 C. 1,240 pounds per square inch 310" C. 4,770 pounds per square inch 320 C. 3.260 pounds per square inch Since resistance to deformation is relative to flow stress, it will be seen that resistance to deformation increases signifcantly up to 3 l0 C. for this alloy.

Referring now to FIG. 1 of the accompanying drawings, the apparatus includes a die box 11 in which is mounted a forming tool comprising a die or mold I2 having relief passages 13. A sealing ring I4 clamps a blank 15 of Zn-Al eutectoid alloy which has been treated to exhibit superplasticity, to the die box 11. The die box 11 is connected by a pipe 16 to a vacuum source and also to a source of compressed air through suitable control valves (not shown).

A heater indicated generally at 17 comprises arrays of ceramic electrical heaters, such as 18, mounted below an aluminum reflector plate 19 and asbestos insulation 20. The heater 17 is suspended on cables 21 passing around pulley wheels 22 and connected to a counterweight 23 such that the heater is easily retractable.

In practicing the invention the blank 15 is mounted as shown, the heater ,17 is lowered and energized to raise the temperature of the blank 15 to 275 C. and then maintain that temperature by a suitable thermostat. The control valves are then operated in known manner to connect the die box 11 to the vacuum source and thereby vacuum form the blank 15 to the shape of the surface of the die 12.

During the vacuum forming operation the control valves are regulated to ensure that the critical strain rate of the material of the blank 15 is not exceeded and the operation normally takes a minimum of about 3 to 4 minutes. When forming is completed the control valves are operated to disconnect the die box 11 from the vacuum source, and the thermostat controlling the heater 17 is overridden to allow the heater to raise the temperature of the formed blank. The clamps around the sealing ring 14 are released and the sealing ring is removed, and when the temperature of the formed blank has reached about 310 C., the control valves are operated to admit a blast of compressed air into the die box 11 to strip the formed blank from the die 12. The heater 17 is then switched off and raised, the formed blank is removed and placed aside in a position where it can cool without further handling, and a fresh blank, which may be preheated, is loaded into the apparatus.

FIG. 2 illustrates a pressure forming apparatus that includes a porous refractory forming tool or die 24 having thermostatically controlled electrical heaters 25 embedded therein ad j'acent the forming surface. The upper surface of the die 24, is provided with a sealing ring 26 and the die 24 is contained within a vented die box 27. A pressure box 28, which has a sealing ring 29 and a flexible pipe 30 connecting the interior of the box 28 to a source of compressed air through suitable control valves (not shown), is mounted on a hydraulically controlled cylinder device 31. Banks of radiant electrical heaters, such as 32 are mounted in the pressure box 28 and are thermostatically controlled.

In operation, a blank 33 of eutectoid zinc-aluminum alloy which had been heat-treated to exhibit superplasticity and which may be partially preformed as shown and also may be preheated is placed on the sealing ring 26. The hydraulic cylinder 31 is then operated to clamp the blank 33 between the sealing rings 26 and 29, and the heaters 25 and 32 are actuated to heat the blank 33.

When the blank 33 reached a temperature of about 250 C, the control valves are operated to supply compressed air at about pounds per square inch to the pressure box 28 thereby slowly forming the blank 33 over a period of about 3 minutes to the shape of the forming surface of the die 24.

When forming is completed the thermostats controlling the heaters 25 and 38 are overridden thereby allowing the temperature of the blank to rise. When the temperature has reached about 310 C. the heaters are switched off, the pressure box 28 is raised by the hydraulic cylinder 31 and the formed blank is stripped from the die 26 by a plurality of pinejectors (not shown) mounted in the upper surface of the die 26.

The formed blank can then be placed aside to cool and a further blank placed in the forming apparatus.

Whatl claim is:

l. A method of deforming a metal alloy workpiece formed of an alloy which exhibits superplastic behavior including the steps of, heating the alloy to within its superplastic tempera- 

2. The method of claim 1 wherein said alloy is a zinc-aluminum eutectoid alloy in the form of a sheet. 